Hydrogen generation apparatuses include a reformer and a carbon monoxide reducer. The reformer causes a steam reforming reaction between a raw material gas and steam with use of a reforming catalyst, thereby generating a reformed gas whose components include hydrogen, methane, carbon monoxide, carbon dioxide, and steam. Here, a hydrocarbon-based fuel such as natural gas or LPG is used as the raw material gas. The carbon monoxide reducer reduces carbon monoxide in the reformed gas with use of a shift conversion catalyst and/or a selective oxidation catalyst.
The natural gas or LPG used as the raw material gas contains sulfur compounds. The sulfur compounds include those originally contained in the raw material gas when extracted and those added to the raw material gas as odorants for leakage detection purposes. These sulfur compounds are, for example, DMS (sulfides), TBM (mercaptans), and THT (thiophenes). If the sulfur compounds are fed to the reforming catalyst, the shift conversion catalyst, or the selective oxidation catalyst, then the catalyst's active site becomes covered, which hinders the catalyst's performance.
For this reason, from the raw material gas to be supplied to a hydrogen generation apparatus in which catalysts are used, the sulfur compounds need to be removed in advance so that the raw material gas will contain almost no sulfur compounds. Major methods used to remove the sulfur compounds from the raw material gas include: an adsorption desulfurization method in which the sulfur compounds are physically adsorbed within a desulfurization agent without being converted; and a hydrodesulfurization method in which the sulfur compounds are reacted with hydrogen, so that the sulfur compounds are converted into hydrogen sulfide, and the hydrogen sulfide is adsorbed to an adsorption catalyst.
For household fuel cell systems using a hydrogen generation apparatus, it is necessary to reduce the maintenance of the apparatus as much as possible so that the hydrogen generation apparatus can be used for 10 years or longer. When the adsorption desulfurization method and the hydrodesulfurization method are compared in terms of desulfurization agent usage, the hydrodesulfurization method requires a smaller amount of desulfurization agent to remove the same amount of sulfur compounds (e.g., the amount of usage of a hydrodesulfurization agent is a fraction of the amount of usage of an adsorption desulfurization agent).
Accordingly, one known desulfurization system for desulfurizing a raw fuel for use in producing a hydrogen fuel for a fuel cell is configured such that, when starting up, the desulfurization system removes sulfur compounds from a raw material by using a normal-temperature desulfurizer, and thereafter, when the desulfurization system has become a state where a hydrodesulfurizer can be used, the desulfurization system uses the hydrodesulfurizer instead of the normal-temperature desulfurizer to remove sulfur compounds from the raw material (see Patent Literature 1, for example).